As for phase-current detection in the case above, a method in which phase-current is detected from the current of a DC power line is conventionally well known (see patent reference 1*).
The conventional structure will be described hereinafter with reference to a circuit diagram shown in FIG. 34. According to an rpm instruction signal (not shown) and the like, control circuit 112 of inverter 123 effects control of switching elements 102 in a manner so as to convert DC fed from battery 101 into AC. The AC is fed to stator winding 104 of motor 111, by which magnet rotor 105 is driven. Diodes 103 form a circulating route of current flowing to stator winding 104. In the explanations given hereinafter, it will be assumed that switching elements 102 are formed of upper-arm switching elements U, V, W and lower-arm switching elements X, Y, Z.
Current sensor 106 detects the current value and sends it to control circuit 112. Control circuit 112 uses the value for calculation of power consumption, judgment for protecting switching elements 102 and positional detection of magnet rotor 105.
FIG. 35 shows waveforms (i.e., U-phase terminal voltage 141, V-phase terminal voltage 142, W-phase terminal voltage 143 and neutral-point voltage 129) in sinusoidal three-phase modulation with a maximum modulation degree of 10%. FIG. 36A shows the ON-period (ON-duty) of upper-arm switching elements U, V, W in one carrier (a carrier cycle) at a phase of around 105° in FIG. 35. The ON-period of upper-arm switching elements U, V and W is evenly shown on the left and right sides from the middle of a carrier cycle. In the figure, a thin solid line represents the ON period of the U-phase; a medium solid line represents the V-phase; and a thick solid line represents the W-phase. According to the length of the ON-period, the phases are herein referred to the maximum ON-period phase (the U-phase, in this case), the intermediate ON-period phase (i.e., the V-phase) and the minimum ON-period phase (i.e., the W-phase).
The ON/OFF state of upper-arm switching elements U, V, W tells that which phase of current is detected by current sensor 106. That is, when only one phase is turned on, the current corresponding to the phase flows; when two phases are turned on, the current corresponding to the remaining phase flows; and when all the three phases are turned on or off, no current flows. The ON/OFF state of upper-arm switching elements U, V, W tells which phase-current is detectable by current sensor 106.
In FIG. 36A, α represents the period with only one phase turned on and β represents the period with two phases turned on, both of which are too short for current sensor 106 to detect the phase current. FIG. 36B shows an example that addresses the inconveniency. First, time δ is determined so that current sensor 106 detects the phase current. Time δ is a uniform value with a margin of time in consideration of various situations. In the left half (i.e., in the beginning) of the carrier cycle of FIG. 36B, the ON-period of the U-phase is increased so that α equals to δ; on the other hand, the ON-period of the W-phase is decreased so that β equals to δ. The correction above allows current sensor 106 to detect current of the U-phase and the W-phase. In FIG. 36B, arrows U and W indicate the phase-current-detectable periods of the U-phase and the W-phase.
Compared to the state with no correction, the phase current has a change in the state having correction. Here will be detailed the change in phase current. For sake of clarity, suppose that stator winding 104 of the motor carries inductance L only and resistance R of zero. Besides, for the purpose of obtaining change in the phase current in a carrier cycle, the description will be given without consideration of induced voltage that has little change in a carrier cycle. In addition, the description will be given on the assumption that there is no change in current on the PWM system in two or three consecutive carrier cycles.
FIG. 37 shows the behavior of the phase current (U-phase current iU, V-phase current iV and W-phase current iW) with no correction in two consecutive carrier cycles. In the two carrier cycles, there is no change in current on the PWM system and the ON-period of each phase has a same pattern in the two cycles. In the period where all the three phases have no ON-period (i.e., in the state shown in FIG. 39A), each phase current has no change. In the period where the U-phase only has the ON-period (see FIG. 39B, where an arrowed solid line shows an increase; an arrowed broken line shows a decrease), U-phase current iU increases, whereas V-phase current and W-phase current decrease; current iU changes twice as much as current iV and iW. In the period, each phase current exhibits a linear change, as is shown by the equation: E=Ldi/dt (where, L represents inductance of the stator winding; E represents DC voltage; i represents current), and di/dt, which represents the rate of change with time of current i, takes constant E/L. In the period where the U-phase and the V-phase have the ON-period (see FIG. 39C), W-phase current iW decreases, whereas U-phase current iU and V-phase current iV increase; current iW changes twice as much as current iU and iV. In the period where all the three phases have the ON-period (see FIG. 40A), each phase current has no change.
FIG. 38 shows carrier cycles with correction provided. The carrier cycle on the left side has the correction shown in FIG. 36B. On the other hand, the carrier cycle on the right side has the correction from the state of FIG. 36B to the state of FIG. 36A. That is, to cancel out the correction of FIG. 36B, the correction is provided in a manner that the ON-period of the U-phase decreases, whereas the ON-period of the W-phase increases. In the period where only the W-phase has the ON-period (see FIG. 40B), W-phase current iW increases, whereas U-phase current iU and V-phase current iV decrease; current iW changes twice as much as current iU and iV.
In the carrier cycle without correction (see FIG. 37), each phase current has a gradual, smooth change. On the other hand, in the carrier cycle with correction (see FIG. 38), increasing U-phase current iU has a noticeable rise in the left carrier cycle and has a decrease in the right; in contrast, decreasing W-phase current iW has a noticeable fall in the left carrier cycle and has a rise in the right. Such unwanted variations in phase current are regarded as a ripple current. The ripple current increases with the amount of correction. The ripple current similarly occurs in other correction methods. In the end of the carrier cycle on the right side, U-phase current iU, V-phase current iV and W-phase current iW have a value the same as each phase current in a carrier cycle without correction. That is, increase/decrease in the phase current throughout two carrier cycles with correction has no difference from that in a carrier cycle without correction, and accordingly, there is no influence on the PWM system. In other words, there is no change in phase voltage and phase current throughout two carrier cycles.
To suppress the ripple current and unwanted effect caused by the ripple, some suggestions have been made. For example, according to the methods disclosed in patent references 2* and 3*, there is no need for correction on the ON-period for phase-current detection, and therefore no noise and vibration caused by the ripple current. As compared to the methods above, employing a single current-sensor decreases parts count; and accordingly, contributes to a compact and lightweight structure with high reliability in vibration-proof or the like. The structure detects maximum current passing through the upper-arm and lower-arm switching elements, protecting the switching elements and the diodes connected in parallel from damage. Besides, the current detected by current sensor 106 is DC fed from battery 101, by which electric power fed from battery 101 can be easily calculated.
In phase-current detection, compared to the structure with two or three current-sensors, the structure with a single current-sensor has the advantage of being compact by virtue of its low parts count and improved reliability in vibration-proof or the like.
However, in a case with a small degree of modulation, the phase current cannot be detected without correction. Correction causes a ripple current. The ripple current has an ill effect, as an electromagnetic force, on the stator winding of the motor, mechanical components and the housing, inviting undesirable noise and vibration. In particular, in a vehicle-mounted electric compressor, manufacturers have a difficulty in disposing a noise-proof box in an effort of reducing the size and weight of the compressor. Decrease in vibration and noise has been a major challenge.
Greater amount of correction causes greater ripple current, increasing vibration and noise. To minimize correction, the amount of correction should be determined on a case-by-case basis, not on a constant basis.
Patent reference 1*: Japanese Unexamined Patent Application Publication No. 2003-189670
Patent reference 2*: Japanese Unexamined Patent Application Publication No. 2004-282884
Patent reference 3*: Japanese Unexamined Patent Application Publication No. 2003-209976